In the post-genomic era, the elucidation of protein functions is the most important area in the field of biology, and the development of therapeutic agents for various diseases, each comprising a protein as an active ingredient, has great potential and possibility in the field of pharmaceutical sciences. In basic studies on protein functions, various techniques have been used, including enhancement of specific protein expression or inhibition (knockdown) of protein expression. Probably, the most simple and reliable technique is to directly introduce a protein into cells. Such a direct delivery technique allows analysis of protein functions without any influence on the transcription-translation pathway. Moreover, effective in vivo protein delivery is very important, e.g., in therapeutic applications such as vaccination or protein formulations. Although various protein-based biopharmaceutical studies have been conducted, the instability of proteins in serum and the lack of a device for protein delivery into the cytoplasm have greatly limited further success (I. M. Tomlinson, Nature Biotech., 2004, vol. 22, p. 521-522). For this reason, many researchers have concentrated their efforts on the development of protein delivery techniques using hydrogels, liposomes, nanotubes, inorganic carriers or the like (K. Y. Lee et al., Prog. Polym. Sci., 2007, vol. 32, p. 669-697; N. W. S. Kam et al., Angew. Chem. Int. Ed., 2006, vol. 45, p. 577-581; I. I. Slowing et al., J. Am. Chem. Soc., 2007, vol. 129, p. 8845-8849), but any protein delivery technique has not yet been developed, which is highly efficient and highly stable in serum, and is also widely applicable.